Add better size hints for notification iterators

src/notification.rs

@@ -117,6 +117,10 @@ impl<'a> Iterator for Iter<'a> {
             _ => unreachable!(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.conn.conn.borrow().notifications.len(), None)
+    }
 }
 
 /// An iterator over notifications which will block if none are pending.
@@ -190,4 +194,8 @@ impl<'a> Iterator for TimeoutIter<'a> {
             _ => unreachable!(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.conn.conn.borrow().notifications.len(), None)
+    }
 }